Fasteners are used to hold the plates of a joint together. By “fastener” we include as examples (but do not limit ourselves to), bolts, stud bolts, rods, rivets, etc. In addition to these fasteners, the present invention also applies to such devices as mine ceiling bolts and truss rods.
The axial tension force in the fastener clamps the plates together, or in the case of truss rods, holds nodes of a structure in some constant relation to one another. The problems in providing a known preload tension in a bolted joint are well known. For example a torque wrench used to tension a bolt even in best case provides only a plus or minus 25 percent error in tension, because of the unknown frictional work in torquing the bolt. Bolt elongation methods which consider the bolt as a very massive stiff spring and relate the tension to the measured bolt elongation via Hooke's Law have been used for many decades. The ultrasonic bolt gauge has been around for about 35 years now, and is one of the primary methods of verifying tension in critical bolts. In ultrasonic determination of bolt load, the increased time of flight of an ultrasonic pulse is used to measure increased tension (and therefore elongation and decreased sonic speed) in the fastener.
In any analysis of bolt tension by elongation, it is necessary to know the bolt grip length, and in particular for the ultrasonic methods, the bolt temperature must be known accurately for good tension measurements to be obtained. The impetus to the present invention was the inconvenience in correcting tension loads obtained ultrasonically for temperature and grip length.
Other tensioning techniques such as hydraulic tensioners and bolt heaters are also prone to large errors and require independent verification of preload tension. Other techniques of actual tension verification such as strain gauges are inconvenient and fragile and also prone to performance reliability problems in the sense that a damaged strain gauge will give a wrong reading but not give any indication or signature that the reading is wrong. The present invention actually records a signature pattern for future reference, so that any damage to the bolt (which is itself the indicator of its tension) will be immediately apparent to the operator of our invention.